A number of different standards for transmitting video signals are currently used around the world. For example, the NTSC standard is currently used in North America and the PAL standard is currently used in Europe. The NTSC standard utilizes 525 horizontal lines of resolution per video frame and each video frame is updated 30 times per second. By contrast, the PAL standard utilizes 625 horizontal lines of resolution per video frame and each video frame is updated 25 times per second. Because of these differences in frame rate and refresh frequency, and other signal variables, equipment designed to be compatible with the North American NTSC standard typically cannot be used with equipment designed for use with the European PAL standard. Therefore, a PAL-capable video cassette recorder/player (VCR) cannot be used with an NTSC-capable television. Likewise, video tapes formatted for playback by an NTSC-capable VCR cannot be played back by a PAL-capable VCR. Other standards, that are similarly incompatible with both the NTSC and PAL formats, are also used around the world.
Digital data can be encoded into video signals formatted in any of these incompatible video transmission formats. For example, a data encoder can encode each line in each video frame of a video signal by modulating a pulse onto the line to define a single bit of data. The data encoder may also encode additional pulses to increase bandwidth. Typically, these pulses are located in the horizontal overscan portion of the video signal outside the viewable area. A data detector decodes the pulses and converts the encoded data into usable digital information. Once decoded, the encoded data can be used to control a child's animated toy, to configure a set-top box, to provide advertising, e-mail, weather warnings, or a myriad of other uses.
In the past, the various video transmission standards used around the world has required that a separate data detector be created for each television transmission signal format. But producing separate detectors for each television format is expensive. To complicate matters even further, many VCRs ard televisions sold in Europe are capable of operating with both PAL and NTSC formatted signals. For example, a dual-standard capable VCR used to playback a PAL-formatted video cassette will output a PAL-formatted video signal. If tie same VCR is used to playback an NTSC-formatted video cassette, the VCR will output an NTSC-formatted video signal. Therefore, a PAL-capable data detector sold in Europe cannot be guaranteed to work with each type of input signal it may receive from a VCR Accordingly, it is highly desirable that the data detector be capable of determining the transmission format of the input video signal and decoding data from the video signal regardless of its transmission format.
It is very difficult to determine the transmission format of a video signal that contains irregularities in signal timing, such as a video signal output from EL VCR. Typical helical-scan VCRs utilize the output from multiple read/write heads to create the output video signal. As the videotape passes over the read/write heads, the VCR "switches" back and forth between the output of each head. In this manner, the VCR creates a video signal by combining the output of several heads. When the VCR "switches" between heads, however, a discontinuity of signal occurs and noise is introduced into the video signal. This "switching noise" is common in the signal output from VCRs, but is not problematic because it is typically outside the viewable area of the displayed video signal. This switching noise is problematic, however, when signal timing characteristics are used to determine the transmission format of a video signal output from a VCR. Additionally, other signal irregularities caused by mechanical-servo inaccuracies and imperfections in the videotape material can cause an improper determination to be made when using signal timing to determine the signal transmission format. If the data detector cannot correctly determine the transmission format of the video signal, it will be unable to decode the data encoded in the video signal and any connected devices that rely on the encoded data will not function properly.
Accordingly, there is a need for a method, system, and apparatus for decoding data encoded in the horizontal overscan portion of a video signal that may be in one of many different transmission formats. There is also a need for a method and system for decoding data encoded in the horizontal overscan portion of a video signal that does not utilize the timing characteristics of the video signal to determine the transmission format. There is a further need for a method, system, and apparatus for decoding data encoded in a video signal that can accurately decode data encoded in a video signal output from a VCR, regardless of its transmission format.